Method for the preparation of composite pigments



Patented Oct. 24, 1939 UNITED STATES PATENT OFFICE METHOD FOR THEPREPARATION OF COMPOSITE PIGMENTS Jersey No Drawing.

Original application August 24,

1935, Serial No. 37,775. Divided and this application November 8, 1938,Serial No. 239,511

3 Claims.

My invention relates to novel methods for the manufacture of pigments,particularly to the manufacture of that type of pigment known ascomposite in which a primary pigment is inti- 5 mately associated withone or more secondary pigments.

The objects of my invention include novel methods for the manufacture ofcomposite pigments which methods possess many advantages over those ofthe prior art.

The present application is a division of my copending application,Serial No. 37,775, filed August 24, 1935, Patent No. 2,176,875, issuedOctober 24, 1939.

Pigments are said to be of the composite type when they consist of twoor more individual components. Thus, lithopone, a zinc sulfide-bariumsulfate composition, is a composite pigment. So also are the well-known,so-called coalesced composite titanium dioxide pigments of the prior artin which there is present besides titanium dioxide, either barium orcalcium sulfate. Titanated lithopone, a tertiary pigment composed oftitanium dioxide, zinc sulfide and barium sulfate is also of thecomposite type. In all composite pigments there is present at least onematerial which may be regarded as the primary pigment. Such a materialpossesses relatively good pigment properties such as, for example,tinting strength, hiding power, opacity, etc. In addition to the primarypigment there is also present in composite pigments at least one othermaterial which may be regarded as a secondary pigment. Such a materialpossesses pigment properties relatively inferior to those of the primarypigment. In composite pigments the function of the secondary pigment isreferred to as that of an extender, a carrier or diluent for the primarypigment, athough, in the present invention, the secondary pigment mayitself possess definite pigment properties.

By primary pigment as used by me to designate a component of a compositepigment prepared according to the herein described method, I refer tothat component which possesses superior pigment properties as comparedwith the other component or components of the composite pigment andwhich, whether it be a simple or composite material, is dispersed as asimple material in one suspension. Thus, the primary pigment componentmay consist of a simple material such as. titanium dioxide, zincsulfide, or it may consist of a co -precipitated composite pigmentprepared by prior art methods, such as titanium di- 55 oxide-bariumsulfate pigment. In this latter example the coprecipitated calcinedcomposite tively good pigment properties, e, g, basic lead.

carbonate, zinc oxide, etc., when these latter are used in conjunctionwith a material having even better pigment properties as, for example,titanium dioxide, zinc sulfide, etc.

For the most part, composite pigments have been prepared in the past bymethods which involve a coprecipitation of the primary and secondarypigments and a subsequent mutual calcination. Lithopone, for example, isprepared by a double decomposition reaction which occurs when solutionsof zinc sulfate and barium sulfide are brought together. The resultingprecipitate of zinc sulfide and barium sulfate is then calcined.Composite titanium dioxide pigments are generally prepared by one of twoalternative procedures: (1) by adding the secondary pigment, e. g.,barium or calcium sulfate, to a titanium solution and precipitating thetitanium-oxygen compounds by hydrolysis; or, (2) by adding a solution ofa salt, the sulfate of which constitutes the secondary pigment, e. g.calcium or barium chloride, to a titanium sulfate solution thus formingthe secondary pigment, in situ, and then precipitating thetitaniumeoxygen compounds by hydrolysis. The mixed precipitates oftitaniumoxygen compounds and secondary pigment are then calcinedtogether. Such methods produce pigments concerning which it has beensaid that the titanium dioxide is precipitated on and coalesced with thesecondary pigment.

Among the reasons for this final or penultimate calcination step may bementioned the following: to effect changes primarily in the crystallinestructure of certain of the components of the pigment; (b) to effect orcomplete a coalescence between the primary and secondary constituents ofthe composite pigment; (c) to dehydrate one or more of the constituents,if they exist in the coprecipitated mixture as hydroxides or hydrates,etc. (d) to eliminate volatile impurities.

As a result of this calcination, the pigment properties of the primarypigment, such as tinting strength, covering power, etc, are developed toan optimum degree. The secondary pigment, on the other hand, isessentially unaltered and unimproved. It is evident, therefore, that theprior art methods involve considerable unnecessary treatment of thesecondary pigment and, as a consequence of this unnecessary treatment,increased manufacturing costs.

I have now discovered novel methods whereby composite pigments such asthose comprising a primary pigment and one or more secondary pigmentsmay be prepared without the necessity of resorting to coprecipitationand mutual calcination. My novel methods are particularly suited to thepreparation of composite pigments superior to those manufactured byprior art methods and at a much lower cost of production. Thesenovelmethods comprise the coflocculation of a mixture consisting of adispersed suspension of a primary pigment and separately dispersedsuspensions of one or more secondary pigments after which thecoflocculated composite pigment is washed, dried and puverized.

By the tem cofiocculation as used by me throughout this description ofmy invention, I mean the mutual coagulation and integration of pigmentparticles from a mixture consisting of a dispersed aqueous suspension ofa primary pigment and one or more separately-dispersed aqueoussuspensions of secondary pigments.

My novel cofiocculation methods of preparing composite pigments areclearly distinguishable from piror art precipitation methods forpreparing composite pigments.

In methods involving precipitation, substasces in substantiallymolecular solution undergo, under suitable conditions, physico-chemicalchanges which result in the formation of a second, usually difiicultlysoluble, phase. Fluocculation, on the other hand, involves only theintegration or coagulation of small particles of a dispersed substancesto larger particles or fiocs. In the former phenomenon the molecules aresaid to be condensed to minute ultra-microscopic crystals from which inturn larger particles are developed by what is known as grain-growth. Inthe latter phenomenon particles which may even be so small as toapproach colloidal dimension are integrated to larger, visiblemacroscopic fiocs. In the former the larger particles result from aphysico-chemical interaction between the ultramicroscopic crystals andthe surrounding solution While in the latter the larger particles resultmerely from an integration and adherence of a number of smallerparticles. Whereas, precipitation implies the transition of substancesfrom the solution phase, through the colloidal phase to the solid phase;fluocculation, as used by me, implies merely an agglomeration of smallparticles to form larger ones.

Hydrolysis, with the formation of hydrous oxides, and doubledecomposition or metathesis, with the formation of insoluble salts, aretypical condensation reactions involving precipitation. Thus, forexample, hydrous titanium oxide is formed by hydrolysis of a titanylsulfate solution while barium sulfate is formed by a doubledecomposition reaction between a solution of a soluble barium compoundand a solution of a soluble sulfate.

In practicing my invention, a dispersed suspension of the primarypigment is first prepared by means of a dispersing agent. Separateindividual suspensions'of the secondary pigment or pigments areprepared, also by means of dispersing agents. These suspensions are thenmixed and coflooculation accomplished by one of two procedures (1) bymeans of an added coagulating agent; (2) as a result of an interactionbetween the dispersing agent selected for the primary pigment and thedispersing agent or agents selected for the secondary pigment orpigments.

The dispersion of the primary and secondary pigments may conveniently beeffected by grinding or milling the individual pigments with a suitablequantity of water, for example, a quantity equal in weight to that ofthe pigment. The dispersing agent may be contained in the Water, eitherin solution if it be water-soluble, or in suspension, if it be insolublein water; or it may be added prior to or during the milling or grindingoperation, as desired.

The function of a dispersing agent is not com pletely understood. It isbelieved that the disintegration of aggregates of primary particleswhich results from the use of a suitable dispersing agent is due to astrong, usually preferential absorption of ions. It has been suggestedthat through the use of such an agent a similar electrical charge isimpinged upon the individual particles of the dispersed or suspendedsubstance, the effect of which is toset up a mutual repulsion betweenthe individual particles, thus preventing coagulation. Furthermore, adispersing agent may effect a reduction in the size of the particles ofa dispersed or suspended material by partially dissolving them. Asubstance may also function as a dispersing agent, although with obviouslimited application, if it succeeds in nullifying or destroying theagglomerating effect of some other substance present in the material.

Although these general hypotheses are not conclusive, they furnish, inmany instances, a guide to the proper selection of the dispersing agentsto be employed in the practice of my invention.

Among the materials which exert a dispersing effect on various primaryand secondary pigments and which are, therefore, useful in the practiceof my invention may be mentioned: the carbonates and hydroxides ofalkali metals, e. g. NaOI-I, NazCOs, K2003; ammonium hydroxide, NHiOH;alkali metal silicates, e. g. NazSiOs; sulfides, e. g. HzS, certaininorganic salts, e. g. PbC12 and. BaClz; acidic halides of triandtetravalent elements, e. g. Fe, Th, Tl, Ce, AZ, Zr, etc. metallic soaps,e. g. those of oleic, stearic, palmitic or lauric acid; the ammoniumsoaps of sufonted aliphatic oils, e. g. the ammonium soap of sulfonatedcastor oil; sulfonated fatty oils or acids, e. g. Turkey red oil;alkylsubstituted aryl sulfonic acids and their salts, e. g.isopropylnaphthalene sulfonic acid and the sodium salt thereof; certainemulsifying agents such as saponin, trihydroxyethylamine, etc.

According to the first alternative method of my invention, any suitabledispersing agents may be employed in preparing the dispersed suspensionof primary pigment and that of the sec.- ondary pigment or pigments, andcoflocculation may be brought about by the addition, to the mixedsuspensions, of a coagulating agent, for example, magnesium sulfate.Strongly acidic or basic substancessuoh as sulfuric acidor sodiumhydroxide are often effective as coagulating agents.

It will be readily seen that the selection of a suitable coagulatingagent will be determined 'volved in preparing the dispersed suspensions.

When added to mixed suspensions which are in a stable condition, itshould function in a manner similar to that of an antipatheticdispersing agent, i. e. neutralize or discharge the stabilizinginfluences of the dispersing agents contained in the mixed suspensions.The most effective coagulating agents are electrolytes and as suchfurnish in solution two types of ions: (a) one which may be adsorbed andcause dispersion, and (b) one which tends to neutralize or discharge thedispersing ion and cause flocculation. When dispersed pigment particlesare coflocculated from mixed suspensions by means of a coagulating agentwhich is an electrolyte, the flocculating power of that electrolyte maybe considered as that concentration which furnishes a suflicientquantity of those ions which tend to neutralize not only the originalstabilizing ions of the dispersing agents but also those of theelectrolyte itself.

Therefore an electrolyte with a strongly adsorbable dispersing orstabilizing ion will necessarily require higher concentration in orderto effect cofiocculation than an electrolyte with a weakly adsorbablestabilizing or dispersing ion.

Briefly stated, a coagulating agent should have a stabilizing ion whichis only slightly adsorbable; that is, the influence of the ion havingthe same charge as that impinged upon the particles of the dispersedsubstances should be small, whereas that of the fiocculating or oppositeion should be great.

Although these considerations are useful in selecting coagulating agentsand, to a certain extent, antipathetic dispersing agents for thepractice of my invention I do not limit myself only to'coagulatingagents or antipathetic dispersing agents selected in accordancetherewith.

The second of the two above noted methods 01' carrying out my inventionis to choose the dispersing agents for the primary pigment and secondarypigment or pigments respectively, so that mutual coflocculation occurswithout the aid of a coagulating agent when the suspensionsconantipathetic dispersing agents, for example,

where a dispersion has been prepared by means of a dispersing agentfurnishing preferentially adsorbed ions, an antipathetic dispersingagent will furnish ions capable of neutralizing or discharging thedispersing ions of the first dispersing agent. The ions of theantipathetic dispersing agent which fulfill this function are usually ofopposite electrical potential. Consequently,

, acidic dispersing agents may be regarded as being antipathetic tobasic dispersing agents and vice versa. For example, a mutualcoflocculation will result when a, suspension of titanium dioxide,prepared by means of the acidic salt, aluminum chloride, is mixed with asuspension of barium sulfate prepared with the basic substance, sodiumsilicate.

In general, as will be readily ascertained from this description of myinvention and particularly from the specific embodiments thereof,dispersion of the primary or secondary pigments may be effected by theuse of only a small amount of dispersing agent. When it is desired tobring about the coflocculation of the composite pigments as a result ofneutralizing or antipathetic properties of the respective dispersingagents, it may be advantageous to use substantially stoichiometricproportions of the agents selected. However, if it be desired tocofiocculate by means of a coagulant such consideration is notimportant.

After the pigment particles have been coflocculated from the mixedsuspensions they may be separated from the supernatant liquor by anysuitable means such as decantation or filtration or both. The dispersingagents as well as the coagulating agents are thus practically completelyremoved. The co-flocculated pigment particles may then be washed toremove any residual soluble dispersing agent, coagulating agent, orreaction products resulting from an interaction between antipatheticdispersing agents, and then dried. In those cases where an insolubledispersing agent had been used the small amount re- -1 maining in thepigment after removal of the supernatant liquor and washing will exertno detrimental effect on the quality of the product. When dried,coflocculated pigments prepared according to the methods of the presentinvention may be readily pulverized in order to render them suitable foruse in the industrial arts.

There is obtained, as a result of my novel coflocculation processes, anintegration of particles of the primary and secondary pigmentsoriginally dispersed in which the particles are intimately associated insuch relationship that, after washing, drying and pulverizing, theypossess pigment properties which are comparable to those possessed bythe coalesced composite pigments of the prior art. There is involved, Ibelieve, in the integration resulting from my cofiocculation process anassociation as intimate and/or an afiinity between the primary pigmentparticles and those of the'secondary pigment or pigments which is atleast as strong as that resulting from the co-precipitation and conjointcalcination methods of the prior art for the preparation of theso-called coalesced composite pigments. For these reasons I employ theterm integrated as descriptive of the association of pigment particlescoflocculated as above described.

This application as filed is a division of my application Serial No.37,775, filed August 24, 1935, Patent No. 2,176,875, issued October 24,1939, the claims of which are restricted to the first method above setforth wherein zinc sulfide is employed as the primary pigment withclaims specific thereto, in which the secondary pigment is bariumsulfate, sodium silicate is the dispersing agent and magnesium sulfateis the coagulating agent.

In said original application, Serial No. 37,775, filed August 24, 1935,Patent No. 2,176,875, issued October 24, 1939, I have broadly claimedthe first of the two methods above described wherein dispersing agentsare employed which are not antipathetic to one another and in which acoagulating agent is used for effecting coflocculation of the dispersedpigment particles. In addition to the broad claims to this processspecific claims are included to the use of titanium dioxide andcomposite titanium dioxide pigments as the primary pigments and bariumsulfate and basic carbonate white lead as the secondary pigments,

ill)

- pensions.

thereof Application Serial No. 239,512, filed November 8, 1938, PatentNo. 2,176,877, issued October 24, 1939, comprises another divisionalapplication of the said original application Serial No. 37,775, filedAugust 24, 1935, Patent No. 2,176,875, issued October 24, 1939, and inthat divisional application- I have broadly claimed therein the secondmethod above set forth wherein antipathetic dispersing agents areemployed for the primary and secondary pigments respectively, and inwhich coflocculation of the dispersed pigment particles takes place uponsimply mixing the dispersed sus- In addition to the broad claims to thisprocess specific claims are included to the use of titanium dioxide,composite titanium dioxide pigments, and zinc sulfide as the primarypigments and barium sulfate, calcium sulfate, and basic carbonate whitelead as the secondary pigments, to aluminum chloride, sodium silicateand hydrogen sulfide as dispersing agents, all as willappear in theclaims thereof.

It is to be specifically understood that no abandonment of any of theclaims not presented in Serial No. 37,775, the original application, nowPatent No. 2,176,875, issued October 24, 1939, or in this, or the otherdivisional application above set forth is to be incurred by the order inwhich the Letters Patent hereon and on the said two divisionalapplications may issue or by the fact that all of the claims are notpresented in a single patent, reservation being hereby specifically madeto present the claims in this and in the two other applications as justabove set forth.

In this application, with the understanding and reservation as above setforth I now present the following specific example; it being understood,of course, that my invention is not in any way to be restricted to thisexample as below given which is for illustrative, but not limitative,purposes.

' EXAMPLE No. I

v Zinc sulfide-barium sulfate pigment lbs. of commercial zinc sulfidepigment are dispersed by milling for about 2 hours in 30 gallons ofwater in the presence of 1.5 lbs. of sodium silicate. 233 lbs. of bariumsulfate, ground natural barytes, are milled with 33 gallons of water inthe presence of 2.66 lbs. of sodium silicate, for about 2 hours.

The two dispersions are then mixed and about 2.4 lbs. of magnesiumsulfate in aqueous solution are added to coflocculate the suspendedparticles of zinc sulfide and barium sulfate. The cofiocculated pigmentparticles are filtered, washed and dried at about 100 C., after which apulverization may be employed.

The product of this example is an integrated composite pigmentcomparable to lithopone of similar composition prepared by prior artmethods.

It will be seen that my methods are admirably suited to the preparationof pigments of any desired predetermined proportions. In determining thedesired proportions of primary and secondarypigments the useto which thecomposite pigment is to be put should be taken into consideration. Ifthe requirements are such as to demand a pigment having a high tintingstrength, good opacity, etc., a relatively larger amount of primarypigment will be used.

Lithopone pigments prepared according to my coflocculation methods arepreeminently adapted for all purposes where at the present time,

lithopones made by prior art methods are used.

The foregoing detailed description has been given for clearness ofunderstanding and no undue limitations should be deduced therefrom,

but the appended claims should be construed as.

pigments thus integrated from the supernatant liquor, and washing,drying and pulverizing the same.

2. A process for the manufacture of a composite zinc sulphide-bariumsulphate pigment which comprises dispersing zinc sulphide in aqueoussuspension by means of a small amount of a dispersing agent, separatelydispersing barium sulphate in aqueous suspension by means of adispersing agent which is not antipathetic to the dispersing agent forthe zinc sulphide, mixing the separately dispersed suspension andcoflocculating the dispersed pigments by means of a coagulating agent,separating the cofiocculated pigments thus integrated from thesupernatant liquor and washing, drying and pulverizing the same.

3. A process for the manufacture of a composite zinc sulphide-bariumsulphate pigment, which comp-rises dispersing zinc sulphide by means ofa small amount of sodium silicate, separately dispersing barium sulphateby means of a small amount of sodium silicate mixingthe two suspensionsand adding thereto magnesium sulphate in aqueous solution, separatingthe coflocculated pigments thus integrated from the supernatant liquorand washing, drying and 'pulverizing the same.

HUGH V. ALESSANDRONI.

